Why would the biosphere evolve this way? What selection pressures would favor the evolution of reproduction requiring three individuals over two? Resistance to inbreeding forced by frequent population bottlenecks? Genetic redundancy that would mitigate frequent exposure to mutagens? Etc.

$\begingroup$Plants are the only terrestrial eukariotes with an alternance of haploid and diploid generations. Animals and fungi don't do it.$\endgroup$
– AlexPSep 17 '18 at 15:21

$\begingroup$@AlexP: I'm including gametes and zygotes in that statement. Those are still organisms.$\endgroup$
– AnonymousSep 17 '18 at 16:13

$\begingroup$Do you want a real triploidity, or it's enough to have three diploid sexes?$\endgroup$
– AlexanderSep 17 '18 at 18:03

1

$\begingroup$Gametes are not organisms. They are specialized cells produced by an organism. Unlike spores, gametes cannot survive and grow on their own; they must fuse with another gamete to fulfill their function. (As everything in biology, this has exceptions. Those are exceptions.) Consider plants, which do exhibit an alternance of generations. Diplod sporophytes produce haploid spores; each spore has the capacity to grow into a complete haploid organism, the gametophyte, which produces gametes; a male and female gamete must fuse in a diploid zygote, from which a new diploid sporophyte grows.$\endgroup$
– AlexPSep 17 '18 at 18:11

$\begingroup$Please ask only one question per post. Once the first question is answered, you can ask the second one based on the result. As it is, your question is too broad.$\endgroup$
– ElmySep 17 '18 at 18:31

4 Answers
4

Note: I did not look at your link before posting this answer. The triple helix system looks quite complicated and somewhat unlikely to evolve naturally from simple chemicals.

Consider the ABC sex determination system for diploid individuals, where A, B and C are chromosomes with a distinct form that can pair up. Each of A, B and C contains a distinctive gene that encodes for a protein necessary for viability, two of which combine to determine the sexual characteristics. Two of the same are too much, the resulting individual is not viable and culled in an embryonic stage. This naturally leads to the forms AB, AC and BC as only existing forms.

Now suppose that the chromosomes also encodes other proteins that, apart from the sexual characteristics, also have a great impact on the morphology of the creatures. For instance, chromosome A holds genes that induce a (far) greater size and territorial protection, while form B induces more muscles and form C promotes intelligence. This leads to:

AB: big, strong and dumb and quite territorial

AC: big, weak, smart and quite territorial

BC: small, strong, smart and not territorial at all

Individuals produce gametes of either kind without differentiation among the gametes of either chromosome type (A, B or C) or source of the individual (AB, AC or BC) and further suppose the actual reproduction is performed by pooling a bunch of gametes in an aquatic environment with sufficient nutrition, like terrestrial fish. The following options present themselves:

asexual reproduction, just the gametes of a single individual. 50% chance of producing what is essentially a clone

bisexual reproduction, the gametes of two individuals of same sex or different sex. Again 50% chance of producing offspring, half of which are essentially clones

trisexual reproduction, the gametes of three individuals, two out of three individuals of the same sex again gives a 50% chance of producing offspring with less likelihood of clones, three individuals of different sex gives a 66% percent chance of producing offspring with less likelihood of clones.

As the latter option, three different sexed individuals pooling their gametes produces more offspring with a balanced mix of sexes this is the favored option, especially if the chances of survival of an embryo or larva are quite slim, but approximately equal for all three sexes, again not unlike terrestrial fish.

In our cells we have mitochondria which do a lot of work producing energy etc, but they were not originally part of the cell. They were a separate organism that evolved.

A similar thing could happen on larger scales, let us say that a species has a normal 2-parent breeding system. However it has also co-evolved with a symbiotic organism that lives inside it and is vital in some way to the life of the species.

When breeding two parents create the new lifeform - however something else needs to provide the symbiant or the new life will not live very long. However when the symbiant moves into the "fruiting" stage which allows them to create the new symbiants this weakens the host. Essentially the host becomes pregnant, but with the symbiants baby.

As a result life tends to revolve around 1 fruiting individual cared for by a larger group with the group members reproducing with each other then the fruitbearer providing the symbiants.

If this symbiosis evolved early enough in the planet and provides a large enough advantage then you could expect this sort of pattern to be very widespread throughout the animal life.

Perhaps very early in this world's evolution, the first RNA could have combined to begin the process, then very early on a third contributor developed. This became the successful combination on this world, and from there all life developed.

As we recognize two reproductive sexes, male and female, your world would have three recognizable reproductive sexes; eg. male, female, and perhaps "somale". Where we each contribute half of our chromosomes to produce our progeny, they could contribute one third, or perhaps one contributes half and the other two contribute one quarter each. Or, perhaps two contribute half each, with one acting as a "catalyst" of some kind.

The reproductive cycle would be interesting! Would the nuclear family have three parents, be polyandrous, or be very similar to our own but with the "somale" members of society drifting in and out of relationships much more casually? If the latter, would the "somale" gender be less likely to occur? What societal status would the "somale" enjoy if that were the case? Would coitus between the three genders need to be simultaneous or just close in time? If simultaneous, I think the "inseminee" would probably have evolved a second sexual opening very early along the timeline, assuming that semen is even involved.

Or, perhaps, they're "egg-layers", in which case the physiology could be different altogether.

On Earth, plants which reproduce sexually sometimes require two separate genders of plant, but sometimes the plant can reproduce with itself, and sometimes the plants are all unisexual but cannot reproduce with themselves.

I won't
there is no selective advantage to it and quite a few disadvantages, it is evolutionary unstable and the simple logistical benefit of only needing to find one mate will drastically favor two sex systems. There is also really no way to get it to evolve, since they had to go through a two sex stage from their unisex ancestors.

You stat with un-differentiate gametes, then you get specialization, but you will never get to three specialized gametes from one unspecialized one without going through a two gamete phase, and there is no advantage to go to three from their.

there is a reason it has never evolved on earth despite ample opportunity.